The invention will be described in the context of an Automatic Vehicle Identification (AVI) system capable of exchanging data codes between an interrogator and a transponder. The AVI field is but one environment in which the inventive concepts described herein can be applied. Systems using batteryless transponders, as well as transponders with batteries, may be used for identifying or locating objects bearing the transponders such as cattle, luggage or other items. Further, a transponder might provide status information regarding the object on which it is located, such as a transponder borne on a car door indicating whether that car door is open. Transponders utilized in the above recognition systems or others may be powered from batteries or from wireless radio frequency (RF) signals.
With respect to AVI systems, generally, the interrogator is provided in a toll booth of a toll road, parking garage or other limited access facility. The interrogator (reader)identifies passing automobiles by sending wireless interrogation signals to a transponder (tag), which would normally be a small, self-contained unit placed, for example, on the dashboard or windshield of the car. In this way the car (or other vehicle or object) can be identified in a speedy and efficient manner. Depending on the use of the system, an account associated with the driver, owner, or other designated person can be debited with an access charge. Compatibility standards for one such AVI system are set out in Title 21, Division 2, Chapter 16, Articles 1-4 of the California Code of Regulations, herein known as the Caltrans specification or Caltrans spec. The AVI equipment for toll collection typically consists of two functional elements: vehicle-mounted transponders and fixed-position interrogators.
Because the transponders typically either derive their operating power from a small battery, or from a received RF signal, the transponders are not normally active. The interrogator will transmit an RF trigger pulse to activate (turn-on) the transponders in approaching cars or other objects. The interrogator may transmit a number of RF trigger pulses at regular intervals to wake up any approaching transponders. Alternatively, the interrogator might send an RF trigger pulse in response to an external stimulus to the interrogator indicating that a transponder is approaching (e.g., light, heat, or magnetic sensors). After a time delay, the interrogator or reader then transmits an encoded signal, referred to as the Polling message or interrogation which, upon detection and decoding by the transponder or tag, provides initial information to the transponder as to which data blocks the transponder should transmit.
For transponders having low power ("sleep") modes as in commonly assigned U.S. patent application Ser. No. 08/021,123, the transponders are typically activated from their sleep modes by the presence of a threshold level of RF energy. Once activated a typical transponder might consume approximately 50 times more power than a transponder in its sleep mode.